Automatic frequency correction (for carrier phase and symbol synchronisation) is required by all digital communications systems. The following describes briefly prior art techniques commonly used for AFC in digital communications systems.
FIG. 1 shows the generic form of an AFC loop. A received signal is compared to a local signal in a phase detector. The output of the phase detector is converted to a frequency estimate, and this is processed by a loop filter whose output is used to control a local frequency reference. In this example, the AFC loop is implemented in a remote terminal (User Equipment or UE) of a communication system, and the UE frequency reference voltage controlled oscillator (VCO) is driven by an analogue signal from a Digital-to-Analogue converter, the loop filter being implemented in the digital domain (covering both hardware and software implementations). The overall performance of the AFC loop is determined by the accuracy with which frequency errors can be estimated. The loop filter can influence characteristics of the overall loop such as response time and output frequency variance; however, the loop filter can not overcome non-ideal characteristics in the frequency estimator. For example, a frequency estimator with a very high variance may require an impractically narrow loop filter which results in unacceptable response times. Also, if the frequency estimate is biased, the control loop will not tend to zero steady-state error, but will reach a steady state condition with a residual frequency error which can not be removed by any loop filter characteristic.
Known schemes for frequency estimators may be divided into two groups: Pilot Signal and Data-Directed Loops. Examples of both these schemes are briefly discussed below.
Pilot Signal
Pilot signals have characteristics that are known a priori by the receiver, thus enabling the receiver to maximise the detection probability for these signals. Pilot signals can take a number of different forms, examples of which (described briefly below) are: Unmodulated Carrier, Frequency Correction Burst, Pilot Channel and Midamble Sequences.
Unmodulated Carrier
Basic digital communications systems sometimes use an unmodulated carrier transmitted along with the modulated data. A receiver typically employs a phase-locked loop (PLL) which attempts to track the phase of the unmoduated carrier. A narrow bandwidth is required to ensure that the PLL does not attempt to track the modulated data sequence. An unmodulated carrier signal represents a narrow band signal and can not benefit from the advantages of frequency diversity offered by wide-band modulated signals; hence, the use of this technique is cannot be considered for modern wide-band communication systems such as wide-band code-division-multiple access (W-CDMA) systems.
Frequency Correction Burst
This technique is similar to the unmodulated carrier technique described above; however, it has been adapted to work within a Time-Division-Multiple-Access (TDMA) system. In this case, the carrier signal is not transmitted continuously, but it is time-multiplexed in a known sequence. By using time-multiplexing, the carrier can be transmitted without modulated data present; hence, improving the frequency estimate for a given carrier power. This technique is used in the GSM mobile system and the frequency correction also doubles up as an initial synchronisation signal since the unmodulated carrier is easy to identify. This technique is still a narrow band solution and not appropriate for wideband modulation systems.
Pilot Channel
A pilot channel signal is transmitted by some CDMA systems. The pilot signal is spread across the full modulation bandwidth in the same method as for the rest of the transmitted data. In general, the amount of transmit power dedicated to the pilot signal is optimised for the system deployment. In typical applications, pilot channels in CDMA systems are transmitted continuously and the pilot is used as a coherent reference for reception of the data.
Midamble Sequences
Midambles are generally transmitted in the centre of the timeslot(s) in TDMA systems. Transmission of the data does not occur at the same time as the transmission of the midamble. The midamble is used as a coherent reference for the reception of the data.
Data-Directed Loops
Unlike pilot signals, data-directed loops use the modulated data to generate the phase estimates. These systems do not require the transmission of pilot signals. These systems fall into two main categories:
Non-Decision-Directed Loops
Non-decision directed loops rely on the statistical average of the received data to generate a phase estimate. Examples of non-decision-directed loops are the ‘Squaring Loop’ and the ‘Costas Loop’ used in basic digital modulation systems; these loops are well known, and need not be described further herein.
Decision-Directed Loops
Decision-directed loops rely on the fact that the received data sequence is known. They are often used where hard-decisions are made at the output of the demodulator. This technique is preferred over non-decision-directed loops due to the fact that the phase estimate produced is unbiased and the performance in low signal-to-noise ratios is also superior.
The Time Division Duplex (TDD) version of the 3rd generation Universal Mobile Telecommunications System (UMTS) as specified by the 3rd Generation Partnership Project (3GPP) does not provide any specific pilot signals with the explicit purpose of frequency correction. The synchronisation channel SCH was designed to be detectable in the presence of a large frequency offset, and as a result it is possible to use this signal to generate frequency estimates under these conditions. In addition, a midamble is transmitted in the centre of each timeslot and it is possible to use the midamble sequence to further generate frequency estimates.
However, neither of the above schemes can obtain the frequency correction accuracy required to have minimal impact on the error rate of the received data in all channel configurations of a TDD UMTS system.
A need therefore exists for a communication system, method and unit for automatic frequency control therein wherein the abovementioned disadvantage(s) may be alleviated.